Lectin-carbohydrate binding is known to play an important role in a number of different cell-cell interactions including those between the oral actinomyces and streptococci that colonize teeth. Galactose- and N- acetylgalactosamine reactive lectins associated with the type 2 fimbriae of Actinomyces viscosus and Actinomyces naeslundii, bind complementary receptors on most Streptococcus oralis isolates as well as some strains of S. sanguis, S. gordonii, and S. mitis. Antigenically distinguishable polysaccharides, each composed of a different, phosphodiester linked repeating oligosaccharide unit have been identified as receptor molecules on three strains of S. oralis, one strain of S. gordonii and two strains of S. mitis. Lectin recognition of these polysaccharides occurs at the reducing end of each oligosaccharide repeating unit and depends on the common presence of internal galactofuranose linked ~16 to Gal~13GalNAc` or GalNAcbeta1 to 3Galalpha. Whereas Actinomyces spp. lectins interact with polysaccharides containing either type of receptor structure, GalNAc- binding lectins such as those of S. gordonii prefer GalNAcbeta1 to 3Galalpha-while certain Gal-binding lectins react best with Galbeta1 to 3GalNAcalpha-containing polysaccharides. The antigenic properties of these polysaccharides appear to depend on structural features of the repeating unit other than those directly involved in lectin recognition. This separation of functional regions is clearly illustrated by the structural and immunochemical properties of the S. gordonii 38 receptor polysaccharide. The oligosaccharide repeating unit of this polysaccharide has non-reducing and reducing ends that are structurally identical to those of the S. mitis J22 and S. oralis 34 oligosaccharides, respectively. Whereas the antigenic properties of the strain 38 polysaccharide closely resemble those of strain J22, the strain 38 lectin receptor activity is similar to that of strain 34.